1. Field of the Invention
This invention relates to an apparatus and method for inspecting and testing a sample, such as an aircraft skin panel, by optical metrology and is particularly, but not exclusively, concerned with such method and apparatus applicable to optical non-destructive testing by shearography for aerospace components.
2. Discussion of Prior Art
Coherent optical techniques such as holography, interferometry, electronic speckle pattern interferometry (ESPI), speckle interferometry, particle image velocimetry (PIV) and shearography are currently being utilised for applications such as non-destructive testing (NDT), vibration analysis; object contouring, stress and strain measurement, fatigue testing, deformation analysis and fluid flow diagnosis. All these techniques have associated drawbacks with performance being to some extent a trade-off against specific disadvantages inherent in the individual techniques.
For example shearograpy has high sensitivity and tolerance to environmental noise but known NDT techniques involving shearography are of limited application because of difficulties in inspecting large areas due to inefficiencies in the laser power available and optical beam expansion and delivery systems. Additional problems are encountered with a relatively low signal to noise ratio.
A shearography system works by generating two laterally displaced images of a test sample. In practice this is achieved using a shearing element of which there are many variants, and imaging optics. When the sample is illuminated using coherent radiation such as visible radiation from a laser, these twin images are modulated by a speckled pattern due to the high coherence of the light. These two images interfere to form a macroscopic speckle pattern, which may be recorded electronically using a charge couple device (CCD) and a frame store. Interferometric images or fringe patterns may be generated by subtracting two speckle patterns of the sheared twin image where the second speckle pattern is recorded after the test specimen has been subjected to a stressing force, such as thermal, pressure or vibration. If an appropriate stressing force is applied defects in the structure of the sample are revealed by highly characteristic "figure of eight" fringes.
In practice the resulting fringe patterns are noisy due to spurious intensity variations and consequently the sensitivity of the shearography technique is reduced. Many techniques have been proposed for suppressing such noise by extracting the phase difference between the sheared images from the interferogrammes. One proposed technique, described in our UK patent application number 9708651.6, involves the phase stepping of the two laterally displaced images of the sample, by stepping the phase of one of the two images by 2.pi./3 during each of the line scans of a camera so that successive lines are incremented in phase to encode temporally, as well as spatially, information about the sample within a single frame. The resulting image may be decoded by running a vertical convolution mask over the image. This technique has the advantages of suppressing a substantial proportion of noise whilst providing single frame analysis. A laser shearing interferometer suitable for providing such interferogramme images is disclosed in our UK patent application 9708651.6 and is further described below in the context of the present invention.
In known applications of shearography for non-destructive testing, a coherent light source is usually a split beam laser. Splitting of the beam allows a larger area of the sample to be tested, but in expanding the beam intensity of the light varies with radius of the circular beam. This variation in intensity of light incident on the sample being tested results in poor quality of the reflected images and hence poor quality interferogrammes.
Furthermore when the sample is to be tested under stress induced by temperature or pressure variations, the known methods generally require that the laser beam is delivered via a fibre optic cable into a sealed chamber containing the sample, and in which the pressure or temperature can be varied. The laser is generally situated outside of the sealed chamber as known lasers used in NDT are susceptible to variations in pressure and temperature changes and may not perform to the required standard when exposed to the temperature or pressure variations required for stressing the sample.
It is generally less desirable to use temperature variations to induce stress changes as it is usually difficult to keep the entire sample at the same required temperature and yet to be able to quickly and accurately alter the temperature in the chamber to a new value.
A known pressure variation stressing technique allows the pressure in a chamber to cycle between two pressure levels. Speckle pattern images are recorded at ambient pressure then again at a lower, pre-determined pressure level. An interferometric image is then generated by subtracting the speckle pattern images recorded at the two different pressures. This technique however can produce poor images if the sample undergoes vibration whilst either speckle pattern image is being recorded.